Magnetic bubble domain devices are well known in the art, and in many of these devices it is necessary to transfer bubble domains from one shift register to another. For example, in a major/minor loop type of memory organization, such as is shown in U.S. Pat. No. 3,618,054, bubble domains are transferred between the input/output major loop and the storage minor loops.
Many devices have been described in the prior art for transferring bubble domains from one shift register to another. In general, a good current controlled transfer switch is one which has very low current amplitude requirements even when small bubble domains are used. Additionally, a good transfer switch should be compatible with conventional propagation elements used to move bubble domains in shift registers, and should be such that its design is compatible with the design used for the propagation elements in the shift registers. Further, a switch which can be used for all transfer functions on the magnetic chip is desirable. A switch capable of symmetrical transfer should operate with the same margins regardless of the direction of transfer. Another important criterion for a good transfer switch is that it provide good margins for switching and be reliable in its operation regardless of the manner in which it is fabricated. Still further, it is desirable that the transfer switch be an integral part of the propagation structure used to move magnetic bubble domains.
In the prior art, various bubble domain transfer switches are described using current carrying overlays to switch bubble domains from one propagation track to another in response to an electrical signal pulse. Typically, the current carrying conductors are designed in a loop configuration so that a current pulse in the conductor will generate a localized magnetic field within the loop. This localized field temporarily adds to the fields of the propagation elements in that region and provides an additional attractive or repulsive force on magnetic bubble domains approaching that region. In this manner, the bubble domain is preferentially attracted or repulsed in order to determine the propagation track along which it will move. A representative example of such a switch is shown in the IBM Technical Disclosure Bulletin, Vol. 15, No. 2, July 1972 at page 703. In that switch, bubble domains arrive at a point of ambiguity where two possible positions are available for subsequent bubble domain movement. Current in a conductor determines which path will be taken by the bubble, thus resolving the ambiguity of the switch.
A replicate type of switch using a current carrying loop is described by Bobeck et al in IEEE Transactions on Magnetics, Vol. MAG-9, No. 3, Sept. 1973, at pages 474-480. In this type of switch, current is used to stretch a bubble domain so that it will transfer to a different propagation channel, while an additional amount of current is used to replicate the stretched domain. This type of switch requires high currents and is difficult to incorporate in a single level metallurgy design.
Another version of the replicate switch described in the previous paragraph has been shown by T. J. Nelson, in AIP Conference Proceedings, 18, 95 (1974). This is an all-permalloy switch in which a current conducting path is comprised of permalloy deposited at the same time as the permalloy propagation elements. Transfer from one propagation channel to another utilizes a current along a straight permalloy path linking the propagation channels. A disadvantage of this transfer switch is that very high currents are required when the bubble diameter is small, of the order of one micron and less in diameter. Physically, this switch is large and thus is not compatible with densely packed major/minor loop designs.
Still another transfer switch employing a current carrying conductor is shown in U.S. Pat. No. 3,876,995. A double loop conductor is used to establish a magnetic field which attracts a bubble toward one propagation track and at the same time establishes another magnetic field tending to repel the bubble from another propagation track. Additionally, this patent shows a transfer switch which uses no current. Instead, two bubble propagation tracks merge at a junction.
A relatively new switch design which is particularly suitable for use with small magnetic bubbles and a rotating magnetic field is described in patent application Ser. No. 709,358 filed July 28, 1976, now abandoned and assigned to the assignee of the present invention. This switch element has propagation paths which generally define the letter "Y". These propagation paths are from one arm of the "Y" to the other arm, from one arm of the "Y" to the stem or base portion, or the reverse where a bubble domain travels from the stem or base of the "Y" to one of the arms of the "Y". In addition to having a "Y" shaped element, there is a current carrying conductor which crosses the stem portion of the "Y" shaped magnetic element. The particular path traveled by bubble domain through the transfer switch is determined by the presence or absence of a current through the conductor. This type of switch is generally referred to as a drive-to-retain switch since it is necessary to apply a current continuously in the conductor in order to keep the bubbles from switching. In other words, the current is turned off when a bubble is to be switched. With drive-to-retain switches, the on-chip power dissipation is considerable.